Bar screens have proven particularly valuable in sorting materials which have unequal dimensions. Wire or punched screens are typically used to sort materials of a granular nature in which all three dimensions are approximately equal. However, many classes of objects, including two of particular commercial interest, wood chips and municipal or industrial trash, are not readily amenable to separation by conventional screening processes.
In the manufacture of paper, logs are reduced to wood chips by chipping mechanisms, and the chips are cooked with chemicals at elevated pressures and temperatures to remove lignin. The chipping mechanisms produce chips which vary considerably in size and shape. For the cooking process, which is known as digesting, it is desirable that the chips supplied have a uniform thickness in order to achieve optimal yield and quality. Ideally, the supplied chips will allow production of a pulp which contains a low percentage of undigested and/or overtreated fibers. Thus, a means is needed to separate chips on the basis of thickness rather than any other dimension. Bar screens have proven particularly adept at separating materials based on a single dimension such as thickness.
Bar screens consist of two sets of generally rectangular bars which are joined together in an array of racks. The two sets of bars are interleaved to form a screening bed. The bed consists of the elongated, rectangular bars and the narrow, rectangular spaces between the bars. Material to be sorted is introduced to the surface of the bed and the bars are caused to oscillate so that when one set of bars is going up, the other set is going down. This oscillatory motion tends to tip wood chips or other relatively small planar objects on edge so that those of a given thickness may slide through the gaps between the bars.
Wood chips not only must be sized but must be cleaned of foreign matter. Because the wood chips are manufactured from logs, they typically are contaminated with sand and dirt. Wood chips are often stored outside and on occasion a lot of chips become heavily contaminated with dirt and sawdust which can adversely affect the quality of the pulp formed from the chips if the contaminants are not removed.
What is needed is a bar screen which not only sizes wood chips for thickness but removes small particulate material from the chips.